154 Messrs. Young, Grerrard, and Jevons on Electrical 



corresponding nearly with the maximum and minimum 

 E.M.F.'s. Since these are times of slack water they should 

 correspond with the mean E.M.F. (i. e. zero E.M.F. i£ due 

 allowance is made for the displacement of the zero by the 

 intrinsic E.M.F. of the electrodes). It appears therefore 

 that the " electromagnetic " tide is about 90° out of phase 

 with the sea tide. 



During the period of observation a variety of weather was 

 experienced, and the corresponding variation in the nature 

 of the curve gives evidence of wave-action. There are, 

 however, disturbances which are also mainly associated with 

 rough weather, but are obviously of too long a period to be 

 produced by wave-motion. They are often of considerable 

 amplitude and may be due to local currents caused by the 

 wind. 



PL II. fig. 5 represents a record obtained with electrodes 

 M x and M 2 which were about 200 yards apart. This record 

 shows even more markedly than fig. 4 the progressive 

 diminution in amplitude of the electrical disturbance corre- 

 sponding with the diminution of the tidal amplitude. The 

 records in this respect agree with the tide~iables which 

 indicate that the spring tide on 22.ix.18 was about 10 per 

 cent, higher than that occurring on 7.ix.l8. The maximum 

 amplitude of about 7 millivolts, or 3 "5 millivolts per 100 yds., 

 would correspond to the maximum E.M.F. (~-e) producible 

 by a tide of about 1*7 knots. As in fig. 4 a phase lag is 

 exhibited between the electrical disturbance and the tide. 



The first 12 hours of the record show very markedly 

 disturbances due to wave-motion. The dotted part of the 

 curve is only an approximate representation of the record, 

 the dots being too irregularly scattered to be joined into a 

 curve. 



PI. II. fig. 6 represents a record taken with two electrodes 

 moored vertically one above the other. The electrodes were 

 lashed on a rope which was held between a sinker on the 

 bottom and a canvas float on the top. The record shows no 

 definite sign of a tidal variation, though faint suggestions of 

 such are perceptible. It should be observed, however, that 

 with electrodes so close together any probable tidal variation, 

 even on the most favourable assumptions, would be small 

 compared with the short period disturbances. These short 

 period disturbances are obviously, largely at least, due to 

 wave-motion. 



The differences of phase mentioned in connexion with 

 PL II. figs. 4 & 5 strongly suggest that in the value of 

 e x (= — e + Cps) the term Gps plays a predominant part and 



